<blockquote>For your effect approach, you could use a status condition and only apply it if the target is weak against it (i.e. checking the value being above 100, or whatever you consider as weakness).</blockquote>
Yeah, that's exactly what I did. I've been able to make it work for one target, but not when there are many. Here's my setup:

To the left, I check the starting object's resistance, and if it's weak, I apply a temporary status effect that activates the HUD ''Weak'' over the target. This setup works well, but only for one target.
To the right, I tried something to make it work for multiple targets. I try to check the combatants and store those who are weak into a Data Key. Then I try to apply the Weak status effect to those targets. However, it doesn't work at all. I probably did something very wrong there, I'm not used to play with Select Data yet.
How could I make my schematic work for several targets?
Thank you!